1. Field of the Invention
The invention relates to a method of fabricating strip solar cells on the basis of copper strips coated on one side with CIS (copper indium diselenide) and a homologous substance, and an apparatus suitable therefor.
2. The Prior Art
At present, the ecologically friendliest but also dearest variant of alternative energy generation is based upon photovoltaic modules. The possibilities of further reducing the costs of currently used solar modules based on monocrystalline and polycrystalline silicon are limited owing to the use of the necessary materials. So-called thin-layer solar cells offer the greatest potential for cost reduction. At required coating thicknesses for the active absorber of at the most 2-4 .mu.m, even the use of such expensive materials as indium, gallium and selenium is of subordinate importance. Hitherto, their low attainable efficiency has impeded the large-scale use of thin-layer solar cells. However, in recent years significant progress has been made in this respect, so that thin-layer solar cells, in particular those based on CuInSe.sub.2 (CIS) with attained modular efficiency degrees of about 10% are at their pilot phase.
However, such degrees of modular efficiency are dependent upon cellular efficiencies of 12-14% which can be ensured only by an exact control of the technological parameters in correspondingly expensive equipment. In-line sputtering, vapor deposition and tempering equipment have conventionally been used for fabricating CIS modules; but the high depreciation costs of such equipment limit the cost reducing potential of these thin-layer technologies. Moreover, because of the geometry of such equipment, only relatively narrow solar panels (usually about 30 cm) with small variations can be fabricated. Larger equipment which would permit an effective production of larger thin-layer modules, is extremely expensive and would require production volumes which at present could not be sold, even on a world-wide basis. An interesting technological starting point for circumventing those problems resides in coating foils with solar cells and the subsequent connection of individual foil sections by an overlapping mounting technique of which an effective variant is described in Japanese Patent 2-244,772 (A). This variant is particularly interesting where metal emitter electrodes are dispensed with. In contrast to Japanese Patent 2-244,722 (A), 100% of the surface of the module aperture could be used in such a variant (see FIG. 1). However, the width of the cell would be limited to about 1 cm because of the resistance of the upper collector electrode made of a transparent conductive oxide.
An economic production of solar cells and modules in such an arrangement would require enormously high production rates in the fabrication of cells in order in the time it takes in conventional planar technology to produce comparable surfaces comparable. In NL-Z: Solar Energy Materials and Solar Cells, Vol. 29, 1993, pp. 163-173, German Patent 4,335,385 as well as German Patent 4,103,291 there are described methods of making ternary semiconductor surface layers in which CIS solar cells are fabricated on flexible strip-like substrates. As disclosed, layers of copper and indium are applied individually or simultaneously by conventional techniques and are subsequently reacted with a third component. Another method based upon the subsequent conversion of oxide starter components in a sulfur or selenium gas flow, is described in European Patent 0,574,716 A1. For improving functional parameters, the fabricated CIS absorber layers may be combined with selective etching steps for the removal of interfering copper selenides or sulfides, as described in Japanese Patent specification JP6-29,560.
None of the currently known CIS technologies can, however, satisfy the high production demands placed upon the mentioned strip technology. Also, the application of current cell structures (FIG. 2) would require the use of expensive molybdenum strips or metal strips coated with molybdenum, as substrates. Moreover, the weak adherence of CIS layers on molybdenum surfaces constitutes a hitherto unsolved problem mitigating against a strip technology subjected to high mechanical stresses. The use of substantially cheaper copper strips as supports for CIS solar cells in which the upper surface of the copper strip substrate would be part of the solar cell would constitute an interesting alternative. This is made impossible, however, because the copper strip substrate constitutes a practically infinite Cu diffusion source. In this connection, an exact control of the composition of the CIS layers, indispensable for effective solar cells, is not possible.